Plasma excitation coil

ABSTRACT

A spiral-like multi-turn coil excites a plasma for treating a workpiece in a vacuum plasma processor. In one embodiment two of the turns have a discontinuity. Each discontinuity has a capacitor connected across it. An RF source drives the coil via a matching network, an inductor connected to one coil excitation terminal and a capacitor connected to another coil excitation terminal. The impedances of the inductors and the capacitors at the RF source frequency and the discontinuity locations are such as to cause a standing wave voltage of the coil to have (1) equal and opposite values at the coil terminals, (2) sudden amplitude and slope changes, slope reversals and polarity reversals at each of the discontinuities, and (3) three gradual standing wave voltage polarity reversals, spaced from each other by 120°. Two of the gradual polarity reversals are azimuthally aligned with the discontinuities. In a second embodiment, one turn has a discontinuity having a series capacitor connected across it. A shunt capacitor is connected between the discontinuity and ground.

This application is a continuation of application Ser. No. 09/539,906filed Mar. 31, 2000.

FIELD OF THE INVENTION

The present invention relates generally to plasma excitation coils and,more particularly, to an excitation coil having at least one capacitanceconnected across a discontinuity between the coil excitation terminals,and to a workpiece processor including such a coil. The invention alsorelates to a method of operating an excitation coil such that a standingwave has a sudden amplitude and slope change, as well as a sudden slopereversal, between the coil excitation terminals.

BACKGROUND ART

One type of processor for treating workpieces with an RF plasma in avacuum chamber includes a coil responsive to an RF source. The coilresponds to the RF source to produce electromagnetic fields that exciteionizable gas in the chamber to a plasma. Usually the coil is on oradjacent to a dielectric window that extends in a direction generallyparallel to a planar horizontally extending surface of the processedworkpiece. The excited plasma interacts with the workpiece in thechamber to etch the workpiece or to deposit material on it. Theworkpiece is typically a semiconductor wafer having a planar circularsurface or a solid dielectric plate, e.g., a rectangular glass substrateused in flat panel displays, or a metal plate.

Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral coil forachieving the above results. The spiral, which is generally of theArchimedes type, extends radially and circumferentially between itsinterior and exterior terminals connected to the RF source via animpedance matching network. Coils of this general type produceoscillating RF fields having magnetic and capacitive field componentsthat propagate through the dielectric window to heat electrons in thegas in a portion of the plasma in the chamber close to the window. Theoscillating RF fields induce in the plasma currents that heat electronsin the plasma. The spatial distribution of the magnetic field in theplasma portion close to the window is a function of the sum ofindividual magnetic field components produced by each turn of the coil.The magnetic and electric field components produced at each point alongthe coil are respectively functions of the magnitude of RF current andvoltage at each point. The current and voltage differ for differentpoints because of transmission line effects of the coil at the frequencyof the RF source.

For spiral designs as disclosed by and based on the Ogle '458 patent,the RF currents in the spiral coil are distributed to produce atorroidal shaped magnetic field region in the portion of the plasmaclose to the window, which is where power is absorbed by the gas toexcite the gas to a plasma. At low pressures, in the 1.0 to 10 mTorrrange, diffusion of the plasma from the ring shaped region producesplasma density peaks just above the workpiece in central and peripheralportions of the chamber, so the peak densities of the ions and electronswhich process the workpiece are in proximity to the workpiece centerline and workpiece periphery. At intermediate pressure ranges, in the 10to 100 mTorr range, gas phase collisions of electrons, ions, andneutrons in the plasma prevent substantial diffusion of the plasmacharged particles outside of the torroidal region. As a result, there isa relatively high plasma flux in a ring like region of the workpiece butlow plasma fluxes in the center and peripheral workpiece portions.

These differing operating conditions result in substantially largeplasma flux (i.e., plasma density) variations between the ring and thevolumes inside and outside of the ring, as well as at different angleswith respect to a center line of the chamber that is at right angles tothe plane of the workpiece holder. These plasma flux variations resultin a substantial standard deviation, i.e., in excess of three, of theplasma flux incident on the workpiece. The substantial standarddeviation of the plasma flux incident on the workpiece has a tendency tocause non-uniform workpiece processing, i.e, different portions of theworkpiece are etched to different extents and/or have different amountsof molecules deposited on them.

Many coils have been designed to improve the uniformity of the plasma.The commonly assigned U.S. Pat. No. 5,759,280, Holland et al., issuedJun. 2, 1998, discloses a coil which, in the commercial embodiment, hasa diameter of 12 inches and is operated in conjunction with a vacuumchamber having a 14.0 inch inner wall circular diameter. The coilapplies magnetic and electric fields to the chamber interior via aquartz window having a 14.7 inch diameter and 0.8 inch uniformthickness. Circular semiconductor wafer workpieces are positioned on aworkpiece holder about 4.7 inches below a bottom face of the window sothe center of each workpiece is coincident with a center line of thecoil and the chamber center line.

The coil of the '280 patent produces considerably smaller plasma fluxvariations across the workpiece than the coil of the '458 patent. Thestandard deviation of the plasma flux produced by the coil of the '280patent on a 200 mm wafer in such a chamber operating at 5 milliTorr is aconsiderable improvement over the standard deviation for a coil of the'458 patent operating under the same conditions. The coil of the '280patent causes the magnetic field to be such that the plasma density inthe center of the workpiece is greater than in an intermediate part ofthe workpiece, which in turn exceeds the plasma density in the peripheryof the workpiece. The plasma density variations in the differentportions of the chamber for the coil of the '280 patent are much smallerthan those of the coil of the '458 patent for the same operatingconditions as produce the lower standard deviation.

Other arrangements directed to improving the uniformity of the plasmadensity incident on a workpiece have also concentrated on geometricprinciples, usually concerning coil geometry. See, e.g., U.S. Pat. Nos.5,304,279; 5,277,751; 5,226,967; 5,368,710; 5,800,619; 5,401,350;5,558,722 and 5,795,429. However, these coils have generally beendesigned to provide improved radial plasma flux uniformity and to alarge extent have ignored azimuthal plasma flux uniformity. In addition,the fixed geometry of these coils does not permit the plasma fluxdistribution to be changed for different processing recipes. While weare aware that the commonly assigned copending U.S. application of JohnHolland for “Plasma Processor with Coil Responsive to Variable AmplitudeRF Envelope,” Ser. No. 09/343,246, filed Jun. 30, 1999, and Gates U.S.Pat. No. 5,731,565 disclose electronic arrangements for at willcontrolling plasma flux uniformity for different recipes, the Hollandand Gates inventions are concerned primarily with radial, rather thanazimuthal, plasma flux uniformity. In the Holland invention, control ofthe plasma flux uniformity is achieved by controlling a variableamplitude envelope the RF excitation source applies to the coil. In theGates invention a switch or a capacitor shunts an interior portion of aspiral-like RF plasma excitation coil.

It is accordingly an object of the present invention to provide a newand improved coil for a vacuum plasma processor and method of operatingsame wherein plasma flux in the processor is relatively uniform.

An additional object of the present invention to provide a new andimproved coil for a vacuum plasma processor and method of operating samewherein the plasma density incident on a workpiece of the processor hasrelatively high azimuthal uniformity.

A further object of the invention is to provide a new and improved coilfor a plasma processor, wherein the amplitude variations of standingwaves (voltages and/or currents) in the coil are substantially reduced.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a coil for a plasmagenerator of a processor for treating a workpiece includes (1) first andsecond RF excitation terminals, (2) sufficient length to exhibittransmission line effects for RF excitation of the coil and (3) acapacitor connected to internal locations of the coil on different sidesof a discontinuity in the coil. The plasma generator includes a chamberhaving an inlet for introducing into the chamber a gas which can beconverted into the plasma. The coil is adapted to be positioned tocouple an RF field to the gas for exciting the gas to the plasma state.

Preferably, the capacitor has an impedance value for the RF excitationsuch as to cause sudden changes at the location of the discontinuity inamplitude, slope and slope direction of an RF standing wave along thecoil.

In the preferred embodiment, (1) the capacitor has an impedance valuefor the RF excitation, (2) the discontinuity has a location, and (3) thecircuitry for supplying the RF excitation to the coil are such that thestanding wave voltage along the coil has a voltage polarity change atthe location of the discontinuity.

The RF excitation circuitry preferably includes another capacitor and amatching circuit in series with an inductor. The series combination ofthe matching circuit and the inductor are connected between a first coilexcitation terminal and an RF source. The other capacitor is connectedbetween a second coil excitation terminal and a reference potentialterminal. The inductor and other capacitor have values for causingapproximately equal magnitude and opposite polarity standing wave RFvoltages to be at the first and second excitation terminals.

The coil preferably includes plural internal discontinuities and acapacitor is connected to the coil across each discontinuity. Each ofthe capacitors has an impedance value for the RF excitation such as tocause along the coil, at the location where each discontinuity islocated, sudden changes in RF standing wave voltage amplitude, slope andslope direction. Each capacitor has an impedance value for the RFexcitation, each discontinuity has a location, and the circuitry forsupplying the RF excitation to the coil are such as to cause thestanding wave voltage to have a voltage polarity change at the locationof each discontinuity.

Preferably the coil includes plural turns. The excitation circuitry andthe locations of the discontinuities are such that standing wave voltagepolarity reversals occur at locations along the coil displaced from thelocations of the discontinuities. The polarity reversals areapproximately at the same azimuth angle of the coil in different ones ofthe turns.

Another aspect of the invention relates to a method of operating a coilthat applies an RF plasma excitation field to an ionizable gas. The RFfield ionizes the gas to the plasma. The coil has transmission lineeffects so there is an RF standing wave along the coil between oppositeRF excitation terminals of the coil. The method comprises (1) applyingan RF excitation voltage to opposite RF excitation terminals of thecoil, and (2) suddenly changing by a substantial amount the RF standingwave amplitude and slope and the RF standing wave slope direction at alocation along the coil between the excitation terminals.

Preferably, the method also includes suddenly changing the RF standingwave amplitude and slope and the RF standing wave slope direction atplural locations along the coil between the excitation terminals. Eachsudden change is such as to reverse the polarity of the RF standingwave.

The RF excitation is preferably applied such that there areapproximately equal magnitude and opposite polarity standing wavevoltages at the opposite RF excitation terminals.

The method is preferably practiced with a coil having plural turns. Themethod preferably includes causing the standing wave to have gradualchanges in at least some of the plural turns and the sudden changesalong at least some of the plural turns. The gradual and sudden polarityreversals along some of the turns preferably are at substantially thesame coil azimuthal angle and in the opposite direction at substantiallythe same azimuthal angle of the coil. A first gradual polarity reversaland a first sudden polarity reversal occur along a first turn, while asecond gradual polarity reversal and a second sudden polarity reversaloccur along a second turn. The first gradual and second sudden polarityreversals are at substantially the same first azimuth angle of the coil,while the second gradual and first sudden polarity reversals are atsubstantially the same second azimuth angle of the coil. The polarityreversals occur at azimuthal angles that are equally displaced from eachother. One of the turns has a gradual polarity reversal at an azimuthangle different from the sudden polarity reversals.

In accordance with one embodiment of the invention, a variable shuntcapacitor is connected to the coil and a variable capacitor is connectedin series across a coil discontinuity. The capacitances of the variablecapacitors are preferably varied to control the standing wave currentand voltage in the two coil segments that are connected together by theseries capacitor. To facilitate such control, one electrode of the shuntcapacitor is preferably connected to an electrode of the seriescapacitor. The shunt capacitor creates a standing wave currentdiscontinuity along the coil without introducing a discontinuity in thestanding wave voltage along the coil. The series capacitor creates astanding wave voltage discontinuity along the coil without introducing adiscontinuity in the standing wave current along the coil.

The previously mentioned Gates patent differs from the present inventionbecause in Gates a capacitor shunts a part of the coil, rather thanbeing connected across a discontinuity of the coil. The Gates patentdoes not indicate the shunt capacitor causes a sudden slope reversal ofa standing wave voltage along the coil. The implication is that theshunt capacitor, which reduces the electromagnetic field in a centerportion of the coil, does not reverse the standing wave voltage slopedirection. If the shunt capacitor had a large enough value to reversethe standing wave voltage slope direction, no RF electromagnetic fieldwould be derived from the shunted portion of the coil and one of thepurposes of FIG. 3 of the Gates patent, i.e., to derive an RFelectromagnetic from a center portion of the coil, would be defeated.Also, the Gates shunt capacitor causes increased current variations indifferent parts of the coil because the current in the coil portion notshunted by the capacitor is responsive to the sum of the currentsflowing out of the capacitor and the coil portion the capacitor shunts.The increased coil current variations have a tendency to produce plasmadensity non-uniformity.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed descriptions of plural specific embodiments thereof,especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a plasma processor including a coilaccording to the present invention;

FIG. 2 is a front view of one preferred embodiment of a spiral-like coilincluding series capacitors in accordance with the present invention;

FIG. 3 includes a waveform plot of the amplitude of the standing wave RFvoltage along the coil of FIG. 2 in alignment with the length of thecoil and the circuitry driving the coil;

FIG. 4 includes waveform plots of standing wave voltage of each turn ofthe coil of FIG. 2, as a function of azimuth angle;

FIG. 5 includes waveform plots of the voltage and current standing wavesversus coil length for a coil similar to the coil of FIG. 2, except thatthe similar coil is continuous with no lumped parameter impedancesconnected between the coil excitation terminals;

FIG. 6 includes waveform plots of standing wave voltage and current as afunction of coil length when the coil of FIG. 2 is driven by thecircuitry of FIG. 1;

FIG. 7 is a plot of current distribution for the coil of FIG. 2, as afunction of azimuthal angle;

FIGS. 8 and 9 are respectively waveform plots of standing wave currentand voltage variations as a function of azimuth angles for a coilsimilar to the coil of FIG. 2, except that the similar coil iscontinuous with no lumped parameter impedances connected between thecoil excitation terminals;

FIG. 10 is a top view of a modification of the coil of FIG. 2; and

FIG. 11 includes waveform plots of the amplitudes of the standing wavecurrent and voltage along the coil of FIG. 10 in alignment with thelength of the coil and the circuitry driving the coil.

DETAILED DESCRIPTION OF THE DRAWING

The vacuum plasma workpiece processor of FIG. 1 of the drawing includesvacuum chamber 10, shaped as a cylinder having grounded metal wall 12,metal bottom base plate 14, and circular top plate structure 18,consisting of a dielectric window structure 19, having the samethickness from its center to its periphery. Sealing of vacuum chamber 10is provided by conventional gaskets (not shown). The processor of FIG. 1can be used for etching a semiconductor, dielectric or metal substrateor for depositing molecules on such substrates.

A suitable gas that can be excited to a plasma state is supplied to theinterior of chamber 10 from a gas source (not shown) via port 20 in sidewall 12. The interior of the chamber is maintained in a vacuumcondition, at a pressure that can vary in the range of 1-100 milliTorr,by a vacuum pump (not shown), connected to port 22 in base plate 14.

The gas in the chamber is excited to a plasma having a spatiallysubstantially uniform density by a suitable electric source. Theelectric source includes a substantially planar coil 24, mountedimmediately above window 19 and excited by RF power source 26, typicallyhaving a fixed frequency of 13.56 MHz.

Impedance matching network 28, connected between output terminals of RFsource 26 and excitation terminals of coil 24, couples the output powerthat RF source derives to the coil. Impedance matching network 28includes variable capacitors 42 and 44 and fixed capacitor 40 connectedin a “T” network so one electrode of series capacitor 44 is connected toan output terminal of RF source 26, while the other terminal ofcapacitor 44 is connected to a common terminal of series capacitor 42and shunt capacitor 40. A controller (not shown) varies the values ofcapacitors 42 and 44 in a known manner to achieve impedance matchingbetween source 26 and a load including coil 24, terminating capacitor80, and the plasma load the coil drives.

Workpiece 32 is fixedly mounted in chamber 10 to a surface of workpieceholder (i.e., chuck) 30; the surface of holder 30 carrying workpiece 32is parallel to the surface of window 19. Workpiece 32 is usuallyelectrostatically clamped to the surface of holder 30 by a DC potentialof a DC power supply (not shown). RF source 31 supplies an RF voltage,usually 13.56 MHz, to impedance matching network 33, that includesvariable reactances (not shown). Matching network 33 couples the outputof source 31 to holder 30. A controller (not shown) controls thevariable reactances of matching network 33 to match the impedance ofsource 31 to the impedance of an electrode (not shown) of holder 30. Theload coupled to the electrode is primarily the plasma in chamber 10. Asis well known the RF voltage that source 31 applies to the electrode ofholder 30 interacts with charge particles in the plasma to produce a DCbias voltage on workpiece 32.

Surrounding planar coil 24 and extending above top end plate 18 is ametal tube or can-like shield 34 having an inner diameter somewhatgreater than the inner diameter of wall 12. Shield 34 decoupleselectromagnetic fields originating in coil 24 from the surroundingenvironment. The distance between shield 34 and the peripheral regionsof coil 24 is large enough to prevent significant absorption by shield34 of the magnetic fields generated by the peripheral regions of coil24.

The diameter of cylindrically shaped chamber 10 is large enough toprevent absorption by chamber walls 12 of the magnetic fields generatedby the peripheral regions of coil 24. The diameter of dielectric windowstructure 19 is greater than the diameter of chamber 10 to such anextent that the entire upper surface of chamber 10 is comprised ofdielectric window structure 19. The distance between the treated surfaceof workpiece 32 and the bottom surface of dielectric window structure 19is chosen to provide the most uniform plasma flux on the exposed,processed surface of the workpiece. For a preferred embodiment of theinvention, the distance between the workpiece processed surface and thebottom of the dielectric window is approximately 0.3 to 0.4 times thediameter of chamber 10; the inner diameter of chamber 12 is 14 inches,the diameter of coil 24 is 12 inches, the inner diameter of cylindricalshield 34 is 14.7 inches, and the distance between the workpieceprocessed surface and the bottom of the dielectric window is 4.7 inches.

Planar coil 24 functions as a transmission line to produce standing wavevoltage and current patterns along the length of the coil. The standingwave patterns result in variations in the magnitude of the peak-to-peakRF voltages and currents along the length of the coil. The dependence ofthe electric and magnetic fields generated by the coil on the magnitudeof these RF voltages and currents results in differing amounts of plasmabeing produced in different portions of chamber 10 beneath differentportions of the coil.

The variations in the RF current magnitude flowing in different parts ofthe coil and of the RF voltages between different parts of the coil andbetween the coil and ground are spatially averaged to assist in derivinga uniform plasma. Spatially averaging these different current values inthe different parts of the coil substantially reduces non-radialasymmetries in the plasma density, particularly at regions of high RFcurrent in the coil segments near the coil periphery. The transmissionline behavior of the RF current in prior art planar coils increases theamount of magnetic flux generated by the peripheral coil segmentsrelative to the center coil segments. In the present invention coil 24is arranged (as described infra) so there is a relatively uniformstanding wave voltage distribution along the coil and a relativelyuniform azimuthal distribution of plasma flux on the workpiece.

The center portion of coil 24 includes first and second excitationterminals respectively coupled by leads 58 and 56 to opposite terminalsof RF source 26 via (1) the series combination of matching network 28,including inductor 46, and (2) one electrode of capacitor 80, the otherelectrode of which is grounded. Inductor 46, having an inductiveimpedance (Z_(L)=j2πfL) at the excitation frequency (f) of source 26,and capacitor 80, having a capacitive impedance Z_(cap)=1/(j2πfC) (wherej={square root over (−)}1, L is the inductance of inductor 46, and C isthe capacitance of capacitor 80), shift the amplitude and location ofthe voltage and current standing waves across the entire length of coil24. The standing wave voltage and current distribution are shifted incoil 24 by inductor 46 and capacitor 80 so the standing wave voltages atthe first and second excitation terminals of the coil are approximatelyequal in magnitude but have opposite polarity. Coil 24 produces RFelectric and magnetic fields which provide substantially uniform plasmaflux on the processed surface of workpiece 32.

The locations of the standing wave voltage and current in coil 24 arecontrolled by selecting the values of inductor 46 and capacitor 80 sothe peak-to-peak RF currents at the coil excitation terminals areapproximately equal and have minimum values. In one preferredembodiment, the inductance of inductor 46 and the capacitance ofcapacitor 80 are selected to achieve Z_(L)=+100j ohms and Zcap=−100johms. At this condition, coil 24 has opposite polarity maximumpeak-to-peak (i.e., standing wave) RF voltages at its first and secondexcitation terminals. The function performed by inductor 46 can beachieved by increasing the inductance of coil 24, and/or by changing thenominal values of capacitor 40, in which cases inductor 46 iseliminated.

As illustrated in FIG. 2 in one preferred embodiment, spiral-like coil24 includes four circular, substantially flat, coaxial turns 101, 102,103 and 104, such that the radius of each circular turn increases as thereference numeral of the turn increases, such that in one exemplary coilturns 101, 102, 103 and 104 respectively have diameters of about 9.5cms, 17 cms, 31 cms and 39 cms. Coil 24 includes two interior excitationterminals 106 and 108, respectively at opposite ends of turns 101 and102. Terminal 108 is connected to the one end of inductor 46, whileterminal 106 is connected to the ungrounded electrode of capacitor 80.

Each of turns 101-104 has an angular extent of approximately 350°, sothat turns 101-104 respectively have gaps 111-114 between opposite endsthereof. Metal strap 116 electrically connects opposite ends of turns102 and 103 to each other, while metal strap 118 electrically connectsopposite ends of turns 103 and 104 to each other. Straps 116 and 118extend in generally the same direction, outwardly from the left end ofthe inner winding to the right end of the outer winding. Metal strap120, which extends in the opposite angular direction from straps 116 and118, electrically connects the right end of turn 101 to the left end ofturn 104. Turns 101-104 and metal straps 116 and 118 are in generallythe same plane which is different from a plane including strap 120.Strap 120 is in a plane farther from window 18 than the plane occupiedby turns 101-104 and straps 116 and 118. Straps 116, 118 and 120traverse a region occupied by gaps 111-114, to assist in minimizingcross coupling of current flowing in the straps and current flowing inturns 101-104.

The capacitances of capacitors 44, 42, 80, 128 and 130 and theinductances of coil 24 and inductor 46 are such that they form a seriescircuit that is approximately resonant to the frequency of source 26.consequently, there is considerably greater RF current flowing in theseries circuit including components 24, 44, 42, 46, 80, 128 and 130 thanflows from source 26 into capacitor 40 and the sum of the voltagesacross coil 24 and inductor 46 is approximately equal to and of oppositepolarity to the sum of the voltages across capacitors 44, 42, 80, 128and 130. Variable capacitor 42, referred to as the tune capacitor, isadjusted to achieve the series resonant condition.

To reduce the total standing wave voltage along coil 24 betweenterminals 106 and 108, discontinuities 124 and 126 (typically havinglengths of about 1 cm) are respectively provided in turns 103 and 104.Discontinuities 124 and 126 are respectively connected to oppositeterminals (i.e., electrodes) of discrete capacitors 128 and 130. Theazimuthal angle of discontinuities 124 and 126 differ from each other byapproximately 120°. Discontinuity 126 is approximately 120° from theintersection of strap 118 and the right end of turn 104, whilediscontinuity 124 is approximately 120° from the intersection of strap118 and the left end of turn 103.

The locations of discontinuities 124 and 126, as well as capacitors 128and 130, in coil 24, and the impedance values of the capacitors at thefrequency of source 26 are such that there are a sudden amplitudechange, sudden slope changes, sudden polarity reversal and sudden changein slope direction of the standing wave voltage along coil 24 at thelocation of each discontinuity. In a preferred embodiment, thecapacitances of capacitors 128 and 130 are equal so each provides animpedance at the frequency of source 26 equal to −j100 ohms. Thestanding wave voltages have equal and opposite values on oppositeelectrodes of each of capacitors 128 and 130. Turn 103 has a gradualstanding wave voltage polarity reversal at approximately the sameazimuth angle as the azimuth angle where discontinuity 126 is located,while turn 104 has a gradual standing wave voltage polarity reversal atapproximately the same azimuth angle as discontinuity 124. Hence, thereare oppositely directed standing wave voltage reversals in adjacentturns 103 and 104 along line 132 that extends radially from center point134 of coil 24 through discontinuity 124, as well as oppositely directedstanding wave voltage reversals in adjacent turns 103 and 104 along line136 that extends radially from center point 134 through discontinuity126; lines 132 and 136 are displaced about 120° from each other andabout 120° from the centers of gaps 111-114.

The 1⅔ turns, defined by all of inner turn 101 (starting at terminal 106which is connected to the ungrounded electrode of capacitor 80) andtwo-thirds of a turn of outer turn 104 to discontinuity 126 has, in apreferred embodiment, a net inductive impedance at the frequency ofsource 26 of +j150 ohms. The two-thirds of a turn of coil 24 betweendiscontinuities 124 and 126, defined by one-third of a turn of turn 103and one-third of a turn of turn 104, has in the preferred embodiment,net inductance at the frequency of source 26 of +j100 ohms. The 1⅔ turnsof coil 24 from discontinuity 124 to terminal 108 (connected to one endof inductor 46) defined by all of turn 102 and two-thirds of turn 103,has an impedance at the frequency of source 26 of +150j ohms.Representative impedance values of capacitors 40, 42 and 44 of matchingcircuit 28 at the excitation frequency of source 26 are respectively−j150 ohms, −j50 ohms and −j100 ohms. Of course, if inductor 46 isexcluded and its function is provided by coil 24 and/or capacitor 40, asdescribed supra, the inductances of coil 24 and/or capacitors 40 and 44differ from the previously stated values.

Using the aforementioned values, FIG. 3 is a plot of the standing wavevoltages along the coil of FIG. 2 as driven by source 26, matchingnetwork 24, inductor 46 and capacitor 80. For purposes of explanation inFIG. 3, there is assumed to be a finite spacing between several portionsof coil 24 and capacitors 128 and 130; in actuality no such spacingexists and the constant standing wave voltages of FIG. 3 in proximity tocapacitors 128 and 130 do not exist. Coil segments 132, 134 and 136respectively correspond to (1) all of turn 102 in series with two-thirdsof turn 103, (2) one-third of turn 103 in series with one-third of turn104, and (3) two-thirds of turn 104 in series with all of turn 101.

Waveform 138 represents the standing wave voltage of matching network28, inductor 46, coil 24 and capacitors 80, 128 and 130, as a functionof position. In FIG. 3, for explanation purposes, the voltage variationsof waveform 138 are vertically aligned with the circuit components whichproduce the voltage variations. Hence, series capacitors 44, 42, 80, 128and 130 respectively cause large, small, medium, medium and mediumsudden decreases of voltage standing waves respectively indicated bynegatively going, vertically extending step waveform portions 140, 142,144, 146 and 148. Waveform portions 140, 142, 144, 146 and 148 arevertically aligned with capacitors 44, 42, 128, 130 and 80. Inductor 46and coil segments 132, 134 and 136 respectively cause gradual upwardlysloping waveform portions 150, 152, 154 and 156. The voltage increasesof waveform portions 150 and 154, respectively associated with inductor46 and coil segment 134, are approximately equal and less than theapproximately equal voltage increases of waveform portions 152 and 156associated with coil segments 132 and 136.

The values of inductor 46 and capacitor 80 are such that the standingwave voltages 158 and 160 at opposite excitation end terminals 106 and108 of coil 24 have approximately equal magnitude with oppositepolarity, so the standing wave voltages of waveform portions 158 and 160are respectively negative and positive. As discussed infra, this resultcan also be achieved by eliminating inductor 46 and increasing theinductance of coil 24 and/or changing the values of capacitor 40. Theinductive impedance values of coil segments 132, 134, 136 at thefrequency of source 26, the capacitive impedance values of capacitors128 and 130 at the frequency of source 26 and the placement of thecapacitors and the relations of voltages 158 and 160 cause each ofgradual sloping wave portions 152, 154 and 156 to have a zero crossingpoint and each of step wave portions 144 and 146 to have a zero crossingpoint. The zero crossing point of wave portion 154 is in strap 118connecting turns 103 and 104 at the mid-point of the length of coil 24between terminals 106 and 108. The zero crossing points of wave portions152 and 156 respectively occur in turns 103 and 104. The gradual zerocrossing point of wave portion 152 in turn 103 is azimuthally alignedalong radial line 136 with the zero crossing point across discontinuity126 and capacitor 130 in outer turn 104, as indicated by step waveportion 146. The gradual zero crossing point of wave portion 156 inouter turn 104 is azimuthally aligned along radial line 132 with thezero crossing point across discontinuity 124 and capacitor 128 in turn103, as indicated by step wave portion 144.

Waveforms 161, 162, 163 and 164 of FIG. 4 indicate the previouslydescribed azimuthal voltage variations in turns 101, 102, 103 and 104,respectively. Intersections 165 and 166 of waveforms 163 and 164 are theazimuthally aligned zero crossovers of turns 103 and 104 along radiallyextending lines 132 and 136, respectively. The intersections of waveform163 with the 0° azimuth angle and of waveform 164 with the 360° azimuthangle is the gradual zero crossing in strap 118.

Inclusion of capacitors 128 and 130 reduces the total standing wavevoltage variation along coil 24 between terminals 106 and 108. Thestanding wave voltages in windings 103 and 104 individually change only65% as much as in a coil that is continuous between its excitationterminals and does not include capacitors 128 and 130. There is anaverage change of 40% of the standing wave voltages in turns 103 and 104compared to a coil that is continuous between its excitation terminalsand does not include capacitors 128 and 130. Capacitors 128 and 130 alsocause a substantial reduction in current variation along the length ofcoil 24 compared to a coil that is continuous between its excitationterminals and does not include capacitors 128 and 130.

Waveforms 170 and 172 (FIG. 5) respectively plot voltage and currentstanding waves for a theoretical coil having the same geometry as thecoil of FIG. 2, but which is continuous between terminals 106 and 108and has no lumped parameter reactances, such as capacitors 128 and 130.Waveforms 170 and 172 are based on a drive circuit that includescapacitor 80, matching circuit 28 and capacitor 80, but excludesinductor 46. Voltage waveform 170 has approximately a straight linevariation from about −1950 volts at one excitation terminal to about+1950 volts at the other excitation terminal. A single zero crossing isat the approximate center point along the length of the theoreticalcoil, i.e., half-way between the coil excitation terminals. Currentwaveform 172 has minima at the excitation terminals at about 65% ofmaximum value which occurs at the approximate center point along thelength of the theoretical coil.

Waveforms 170 and 172 are quite different from waveforms 176 and 178(FIG. 6) which respectively represent the standing wave voltage andcurrent variations along the length of coil 24, with capacitors 128 and130 included. Standing wave voltage waveform 176 has amplitudes of about−900 volts and +900 volts at excitation terminals 108 and 106,respectively, and intermediate peak values of about±500 volts. Standingwave current waveform 178 has at terminals 106 and 107 minimum values ofabout 88% of peak value. There are three approximately equal peak valuesin standing wave current waveform 178, one in the coil center point 180,i.e., half-way along the coil length between its terminals 106 and 108,a second at point 182 about one-quarter of the coil length from terminal106 and a third at about one-quarter of the coil length from terminal108. Standing wave current minima occur at points 186 and 188 wherecapacitors 128 and 130 are respectively located.

Waveforms 191, 192, 193 and 194 (FIG. 7) are respectively plots of thestanding wave current variations in turns 101, 102, 103 and 104 of coil24 including capacitors 128 and 130 as a function of azimuth angle.Waveforms 193 and 194 indicate that the standing wave currents inadjacent outer turns 103 and 104 are almost the same. The currentdistributions of FIG. 7 are to be compared with the substantialvariation of the current distributions of waveforms 201, 202, 203 and204 of FIG. 8 for the theoretical coil identical to coil 24, but havingno discrete reactances and no discontinuites between its excitationterminals. The relatively low azimuthal standing wave voltage variations161-164 of FIG. 4 are to be compared with the relatively large standingwave voltage variations of waveforms 206, 207, 208 and 209 of FIG. 9 forturns 101, 102, 103 and 104 of the theoretical coil having the sameconfiguration as coil 24, but having no discrete reactances and nodiscontinuities between its excitation terminals.

Waveforms 176 and 178 indicate that the standing wave voltages andcurrents are balanced at the excitation terminals and at intermediatepoints along coil 26. The standing wave voltages and currents areconsidered to be balanced if the voltages at the coil terminals areequal in magnitude and opposite in polarity and if the currents at thecoil terminals are equal in magnitude and polarity. In the theoreticalcoil of FIG. 5, having no discontinuities, such balancing occurs onlyonce, as indicated by (1) the single peak of the standing wave currentwaveform 172 at the approximate mid-point of the coil, and (2) thecontinuous upward slope of standing wave voltage waveform 170. Standingwave voltage and current waveforms 176 and 178 of FIG. 6 indicate thediscontinuous coil of FIG. 2 has three balanced segments; the first,second and third balanced segments are respectively in the ranges ofabout 60120 cms, 120-180 cms and 180-240 cms from terminal 106. Thefirst, second and third balanced segments have approximately equalminimum currents at the beginning and end points thereof, and maxima182, 180 and 184 approximately at the centers thereof. Each of thefirst, second and third balanced segments has an approximately equalnegative standing wave voltage of about −400 v. at the beginning thereofand an approximately equal positive standing wave voltage of about +400v. at the end thereof and a positive going zero crossing standing wavevoltage in its center. These plural approximately balanced segments ofthe coil of FIG. 2 enable relatively uniform plasma density to beachieved, both radially and azimuthally. Because coil 24 includingcapacitors 128 and 130 has voltage and current standing waves withsmaller variations in magnitude along the coil length, greater plasmaflux uniformity, particularly as a function of azimuth angle, isachieved than is the case for the theoretical coil. Arranging thevoltage nulls, i.e., zero crossovers or polarity reversals, to bealigned along radially extending lines 132 and 136, also contributes toazimuthal plasma flux uniformity.

FIG. 10 is a top view of a modification of the coil of FIG. 2 whichincludes four concentric circular turns 235-238, coaxial with centerpoint 211, such that the turns have progressively increasing radii asthe reference numerals associated with them increase. Each of turns235-238 has an angular extent of about 350° so there is a gap of about10° in each of them. The coil of FIG. 10 includes interior and exteriorexcitation terminals 213 and 215, respectively connected to oneelectrode of capacitor 42 of matching circuit network 28 and theungrounded electrode of capacitor 80, which is variable when used withthe coil of FIG. 10. Inductor 46 is not included in the excitationcircuit of the coil of FIG. 10. Terminals 213 and 215 are at oppositeends of turns 235 and 238. Metal strap 217 connects opposite ends ofturns 235 and 236 together, while metal strap 218 connects opposite endsof turns 236 and 237 together. Thus, the coil of FIG. 20 is continuousfrom terminal 213 to the end of turn 237. The coil of FIG. 10 includes adiscontinuity 219 between opposite ends of turns 237 and 238. The coilof FIG. 10 can be considered as having a first segment including turns235, 236 and 237, and a second segment including turn 238.

Variable capacitor 221 bridges discontinuity 219, to provide a seriesconnection of turns 235, 236 and 237 to turn 238. Variable shuntcapacitor 223 is connected between the common terminals at the end ofturn 237 and capacitor 221 and ground. The coil of FIG. 10 could bemodified to include one or more additional discontinuities having seriescapacitors connected across them and one, or more additional shuntcapacitors.

Varying the value of capacitor 223 controls the amplitudes of thestanding wave currents in the two segments of the coil of FIG. 10.Capacitor 223 causes a sudden decrease in the standing wave currentflowing in the second coil segment compared to the standing wave currentin the first coil segment. Increasing and decreasing the value ofcapacitor 223 respectively cause decreases and increases in the shuntimpedance across capacitor 223 and corresponding changes in the ratio ofthe amplitudes of the standing wave currents and voltages in the firstand second coil segments. Variable capacitors 80, 221 and 223 enablebalancing of the standing wave currents and voltages in the first andsecond coil segments. Balancing of the standing wave current in thefirst coil segment occurs when the standing wave currents at the endpoints of the first segment have about the same amplitude and polarity,with a peak current value about half-way between these end points.Balancing of the standing wave voltage in the first coil segment occurswhen the standing wave voltages at the end points of the first segmentare the same in amplitude and opposite in polarity with a zero voltagevalue occurring about half-way between these end points. Balancing ofthe standing currents and wave voltages in the second coil segmentoccurs under similar circumstances for the end points of the second coilsegment.

FIG. 11 includes plots of standing wave current and voltage waveforms212 and 214, along the length of the coil of FIG. 10. Standing wavecurrent waveform 212 includes segments 216 and 218, respectively alongthe first and second coil segments. Each of segments 216 and 218 has apeak value approximately at the center thereof and equal minimumamplitudes and like polarity at the opposite ends thereof. A sudden,almost step decrease 220 in the standing wave current occurs at shuntcapacitor 223. Standing wave voltage waveform 214 includes upwardlydirected segments 222 and 224 along the first and second coil segments,respectively. Segments 222 and 224 are separated by a sudden almost stepdecrease 226 across series capacitor 221. Segment 222 has a greaterslope than segment 224 because more current flows through the first coilsegment than the second coil segment. Segment 222 has equal amplitudeand opposite polarity voltages at opposite ends thereof and a zerocrossing approximately at its center. A similar situation exists forsegment 224. Because of shunt capacitor 223, the standing wave voltagedrop across capacitor 221, indicated by step 226, is not symmetricalwith the zero standing wave voltage value. The positive voltagemagnitude at the end of turn 237 exceeds the negative voltage magnitudeat the beginning of turn 238.

Because the standing wave currents and voltages in the first and secondcoil segments are balanced, the electromagnetic fields the coil of FIG.10 applies to the plasma can be controlled to assist in achievingrelatively uniform plasma density.

While there have been described and illustrated plural specificembodiments of the invention, it will be clear that variations in thedetails of the embodiment specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims.

We claim:
 1. A coil for a plasma generator of a processor for treating aworkpiece, the plasma generator including a chamber having an inlet forintroducing into the chamber a gas which can be converted into theplasma, the coil being adapted to be positioned to couple an RF field tothe gas for exciting the gas to the plasma state, the coil comprisingfirst and second RF excitation terminals, and a capacitor connected tointernal locations of the coil on different sides of a discontinuity inthe coil.
 2. The coil of claim 1 wherein the capacitor has an impedancevalue for the RF excitation such as to cause sudden changes at thelocation of the discontinuity in amplitude, slope and slope direction ofan RF voltage along the coil.
 3. The coil of claim 2 further includingcircuitry for supplying the RF excitation to the coil, wherein (a) thecapacitor has an impedance value for the RF excitation, (b) thediscontinuity has a location, and (c) the circuitry for supplying the RFexcitation to the coil are such that the voltage along the coil has avoltage polarity change at the location of the discontinuity.
 4. Thecoil of claim 3 wherein the circuitry for supplying the RF excitation tothe coil includes another capacitor and a matching circuit, the matchingcircuit being connected between a first coil excitation terminal and aterminal for connection to an RF source, the another capacitor beingconnected between a second coil excitation terminal and a referencepotential terminal, the circuitry for supplying the RF excitation havingvalues for causing approximately equal magnitude and opposite polarityRF voltages to be at the first and second excitation terminals, the coiland capacitor connected to the coil causing equal magnitude and oppositepolarity RF voltages to be at intermediate locations along the coil. 5.The coil of claim 4 wherein the circuitry for supplying the RFexcitation includes an inductor in series with the matching circuit. 6.The coil of claim 1 wherein the coil includes plural internaldiscontinuities, a capacitor being connected to the coil across eachdiscontinuity.
 7. The coil of claim 6 wherein each of the capacitors hasan impedance value for the RF excitation such as to cause in the coil atthe location where each discontinuity is located sudden changes in RFvoltage amplitude, slope and slope direction.
 8. The coil of claim 7further including circuitry for supplying the RF excitation to the coil,wherein (a) each capacitor has an impedance value for the RF excitation,(b) each discontinuity has a location, and (c) the circuitry forsupplying the RF excitation to the coil are such as to cause the voltageto have a voltage polarity change at the location of each discontinuity.9. The coil of claim 8 wherein the coil includes plural turns, theexcitation circuitry and the locations of the discontinuities being suchthat voltage polarity reversals occur at locations in the coil displacedfrom the locations of the discontinuities, the polarity reversals beingapproximately at the same azimuth angle of the coil in different ones ofthe turns.
 10. The coil of claim 1 further including a capacitor inshunt to a reference potential at an intermediate location along thecoil.
 11. The coil of claim 10 wherein the intermediate location is atthe discontinuity.
 12. The coil of claim 11 wherein both of thecapacitors are variable.
 13. A method of operating a coil that appliesan RF plasma excitation field to an ionizable gas, the RF field ionizingthe gas to the plasma, the method comprising applying an RF excitationvoltage to opposite RF excitation terminals of the coil, and suddenlychanging by a substantial amount the amplitude and slope of the voltageand slope direction of the voltage at a location along the coil betweenthe excitation terminals.
 14. The method of claim 13 further includingsuddenly changing the RF voltage amplitude and slope and the RF voltageslope direction at plural locations along the coil between theexcitation terminals.
 15. The method of claim 14 wherein each of suddenchanges is such as to reverse the polarity of the RF voltage.
 16. Themethod of claim 15 wherein the RF excitation is applied such that thereare approximately equal magnitude and opposite polarity voltages at theopposite RF excitation terminals and at locations along the coil betweenthe opposite excitation terminals.
 17. The method of claim 13 whereinthe RF excitation is applied such that there are approximately equalmagnitude and opposite polarity voltages at the opposite RF excitationterminals and at locations along the coil between the oppositeexcitation terminals.
 18. The method of claim 14 wherein the coil hasplural turns, further including causing the voltage to have gradualchanges in at least some of the plural turns and the sudden changes inat least some of the plural turns, the gradual and sudden polarityreversals in some of the turns being at substantially the same coilazimuthal angle.
 19. The method of claim 18 wherein the gradual andsudden polarity reversals occur in the opposite direction atsubstantially the same coil azimuthal angle.
 20. The method of claim 19wherein a first gradual polarity reversal and a first sudden polarityreversal occur along a first turn, and a second gradual polarityreversal and a second sudden polarity reversal occur along a secondturn, the first gradual and second sudden polarity reversals being atsubstantially the same first azimuth angle of the coil, the secondgradual and first sudden polarity reversals being at substantially thesame second azimuth angle of the coil.
 21. The method of claim 20wherein the polarity reversals occur at azimuthal angles that areequally displaced from each other.
 22. The method of claim 21 whereinone of the turns has a gradual polarity reversal at an azimuth angledifferent from the sudden polarity reversals.
 23. The method of claim 13wherein the sudden change is such as to change the polarity of the RFvoltage.
 24. The method of claim 23 wherein the RF excitation is appliedsuch that there are approximately equal magnitude and opposite polarityvoltages at the opposite RF excitation terminals and at locations alongthe coil between the excitation terminals.
 25. The method of claim 13wherein the RF excitation is applied such that there are approximatelyequal magnitude and opposite polarity voltages at the opposite RFexcitation terminals and at locations along the coil between theopposite excitation terminals.
 26. A vacuum plasma processor fortreating a workpiece with a plasma comprising a chamber having an inletfor introducing into the chamber a gas which can be converted into theplasma for treating the workpiece, a coil positioned to couple an RFfield to the gas for exciting the gas to the plasma state, the coilhaving first and second RF excitation terminals, the coil being arrangedso there are substantial sudden changes in amplitude, slope, and slopedirection of the voltage at a location along the coil displaced from theexcitation terminals.
 27. The processor of claim 26 further includingcircuitry for supplying the RF excitation to the coil, the circuitry forsupplying the RF excitation to the coil and the coil being such that thevoltage has a polarity change at said location.
 28. The processor ofclaim 27 wherein the circuitry and the coil are arranged so there areplural substantial sudden changes in the RF voltage amplitude, slope,and slope direction, each of the plural substantial changes being at adifferent location along the coil displaced from the excitationterminals.
 29. The processor of claim 28 wherein the voltage has asudden polarity change at each of said locations.
 30. The processor ofclaim 29 wherein the coil includes plural turns, the coil and theexcitation circuitry being arranged for causing the voltage to havegradual changes along at least some of the plural turns and the suddenchanges in at least some of the plural turns, the gradual and suddenpolarity reversals along some of the turns being at substantially thesame coil azimuthal angle.
 31. The processor of claim 30 wherein thecoil and the excitation circuitry are arranged for causing gradual andsudden polarity reversals to occur in the opposite direction atsubstantially the same coil azimuthal angle.
 32. The processor of claim31 wherein the coil and the excitation circuitry are arranged forcausing a first gradual polarity reversal and a first sudden polarityreversal to occur along a first turn, and a second gradual polarityreversal and a second sudden polarity reversal to occur along a secondturn, the first gradual and second sudden polarity reversals being atsubstantially the same first azimuth angle of the coil, the secondgradual and first sudden polarity reversals being at substantially thesame second azimuth angle of the coil.
 33. The processor of claim 32wherein the coil and the excitation circuitry are arranged for causingpolarity reversals at azimuthal angles that are equally displaced fromeach other.
 34. The processor of claim 33 wherein the coil and theexcitation circuitry are arranged for causing one of the turns to have agradual polarity reversal at an azimuth angle different from the suddenpolarity reversals.
 35. The processor of claim 27 wherein the coil has adiscontinuity at each of the locations, a capacitor being connected toopposite sides of the discontinuity at each of the locations.
 36. Theprocessor of claim 26 wherein the coil has a discontinuity at thelocation, a capacitor being connected to opposite sides of thediscontinuity at the location.
 37. The processor of claim 36 furtherincluding a capacitor in shunt to a reference potential at anintermediate location along the coil.
 38. The processor of claim 37wherein the intermediate location is at the discontinuity.
 39. Theprocessor of claim 38 wherein both of the capacitors are variable.
 40. Acoil for a plasma generator of a processor for treating a workpiece, theplasma generator including a chamber having an inlet for introducinginto the chamber a gas which can be converted into the plasma, the coilbeing adapted to be positioned to couple an electromagnetic field to thegas for exciting the gas to the plasma state, the coil comprising firstand second RF excitation terminals, a winding coupled with the terminalsso the winding is adapted to have current flowing in it in response toAC excitation being applied to the excitation terminals, the windingbeing arranged so the current is adapted to cause excitation of theelectromagnetic field, and a capacitor connected between the terminalsand in series with the winding so the same current adapted to flow inthe winding is also adapted to flow in the capacitor.
 41. The coil ofclaim 40 wherein the capacitor is connected between opposite terminalsof a discontinuity in the interior of the winding.
 42. The coil of claim41 in combination with a chamber.
 43. The coil of claim 41 wherein thecapacitor has an impedance value for the AC excitation such as to causesudden changes at the location of the discontinuity in amplitude, slopeand slope direction of an AC standing wave along the coil.
 44. The coilof claim 41 wherein the winding includes plural internaldiscontinuities, a capacitor being connected to the winding across eachdiscontinuity.
 45. The coil of claim 44 wherein each of the capacitorshas an impedance value for the AC excitation such as to cause in thecoil at the location where each discontinuity is located sudden changesin AC standing wave voltage amplitude, slope and slope direction. 46.The coil of claim 40 in combination with a chamber.
 47. A coil for aplasma generator of a processor for treating a workpiece, the plasmagenerator including a chamber having an inlet for introducing into thechamber a gas which can be converted into the plasma, the coil beingadapted to be positioned to couple an electromagnetic field to the gasfor exciting the gas to the plasma state, the coil comprising first andsecond AC excitation terminals, a winding coupled with the terminals sothe winding is adapted to have current flowing in it in response to ACexcitation being applied to the excitation terminals, the winding beingarranged so the current is adapted to cause excitation of theelectromagnetic field, and a capacitor connected in series with thewinding so the same current adapted to flow in the winding is alsoadapted to flow in the capacitor, the capacitor being connected betweenopposite terminals of a discontinuity in the interior of the winding.48. The coil of claim 47 in combination with a chamber.
 49. The coil ofclaim 47 wherein the capacitor has an impedance value for the ACexcitation such as to cause sudden changes at the location of thediscontinuity in amplitude, slope and slope direction of an AC standingwave along the coil.
 50. The coil of claim 47 wherein the windingincludes plural internal discontinuities, a capacitor being connected tothe winding across each discontinuity.
 51. The coil of claim 50 whereineach of the capacitors has an impedance value for the AC excitation suchas to cause in the coil at the location where each discontinuity islocated sudden changes in AC standing wave voltage amplitude, slope andslope direction.